Abstract: Coarse clastic reservoirs,one of the most commonly seen reservoir types in the world,feature in strong heterogeneity and are highly sensitive to diagenesis process.One of the key research areas for the reservoir is the study of the impact of diagenesis upon its storage capacity.The coarse clastic reservoirs in the slope delta fan of the Lower Triassic Bai-kouquan Formation (T₁b) in Aihu oilfield (an exploration breakthrough was made recently) in western slope of the Mahu Depression in Juggar Basin were chosen to study the diagenesis types and their impact upon storage capacity of the reservoirs.The result indicates that there were mainly four types of diagenese process in the area:mechanical compaction,mineral replacement (albitization of detrital feldspar and illitization of smectite),dissolution (K-feldspar and calcite,etc.) and cementation (calcite,kaolinite and small amounts of chlorite).These processes had significant influence upon the quality of the reservoirs.Compaction undermined the primary intraparticle pores and soured physical properties of the layers.Mineral replacement had only limited impact over the properties; but relatively speaking,dissolution of minerals like potash feldspar and etc.facilitated by hydrocarbon-bearing fluids migrating along faults greatly increased storage capacity of the layers by forming secondary pore zones in coarse clastic rocks with fair primary physical properties such as underwater channel sands of fan delta fronts and granular flow deposits.Cementation was destructive to the preservation of pores formed during the middle and late diagenetic stages,and the local enrichment of kaolinite and calcite caused the formation of low-permeability layers.The understanding was confirmed by various reservoir properties in different members of the Formation:high-quality reservoirs were found in T₁b₁ near faults connecting source rocks,high-to good-quality reservoirs were found in T₁b₂,at an updip position adjacent to the faults connecting source rocks,but almost no reservoirs were found in T₁b₃.

Key words: diagenesis, reservoir, physical property, coarse clastic, Baikouquan Formation, Mahu Sag, Junngar Basin